buildbot: Enable merging Mac output.

testing/buildbot/chromium.fyi.json

 {
   "Android Asan Builder Tests (dbg)": {
     "gtest_tests": [
       {
         "args": [
           "--tool=asan"
         ],
         "swarming": {
           "can_use_on_swarming_builders": false
         },
@@ skipping 12047 matching lines @@
           "hard_timeout": 6000,
           "shards": 15
         }
       }
     ]
   },
   "WebKit Mac - RandomOrder": {
     "isolated_scripts": [
       {
         "isolate_name": "webkit_layout_tests_exparchive",
+        "merge": {
+          "args": [
+            "--verbose",
+            "--results-json-override-with-build-property",
+            "build_number",
+            "buildnumber",
+            "--results-json-override-with-build-property",
+            "builder_name",
+            "buildername",
+            "--results-json-override-with-build-property",
+            "chromium_revision",
+            "got_revision_cp"
+          ],
+          "script": "//third_party/WebKit/Tools/Scripts/merge-layout-test-results"
+        },
         "name": "webkit_layout_tests",
+        "results_handler": "layout tests",
         "swarming": {
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
               "os": "Mac-10.9"
             },
             {
               "os": "Mac-10.12"
             },
             {
@@ skipping 280 matching lines @@
         "test": "views_unittests"
       }
     ]
   },
   "Win Builder (ANGLE)": {
     "additional_compile_targets": [
       "all"
     ]
   }
 }